Female Microplitis demolitor (Wilkinson) (Braconidae: Hymenoptera) were attracted to Tenax-trapped volatiles from the larval frass of soybean looper, Pseudoplusia includens (Walker) (Lepidoptera: Noctuidae), produced on the foliage of looper-susceptible Davis and looper-resistance PI 227687 soybeans and host Henderson lima beans, in an olfactometer. Linalool (3,7-dimethyl-1,6-octadien-3-ol), guaiacol (2-methoxyphenol), and 3-octanone were identified as major components of such frass volatiles by GC-MS analysis. These compounds were not detected in the volatiles of frass produced on an artificial diet. Such volatiles produced on the artificial diet were not attractive to the parasitoid. Authentic samples of 3-octanone and guaiacol were attractive to female M. demolitor in olfactometer tests. These two compounds also elicited higher EAG responses in female M. demolitor than linalool and cis-3-hexen-1-ol. The quantity of guaiacol in the soybean looper frass produced on PI227687 was higher (ca. 5 times) than on Davis soybean or lima bean leaves. 3-O-Octanone was detected in the volatiles trapped from excised foliage of Davis and excised foliage and intact plants of PI 227687 soybeans. Guaiacol and linalool were not detected in soybean foliage volatiles. The significance of volatiles in the host-seeking behavior of M. demolitor, the tritrophic interactions among plants, herbivores, and natural enemies, and the role of herbivore frass volatiles in maintaining the natural enemy's host specificity are discussed. none